The code unit, which operates with was designed to control an electromagnetic door lock with a four-digit code. It resets automatically after a short period.
The keys that set code are designated S1-84 in the diagram. Each of them is linked to a bistable (flip-flop) in IC1. The connections are arranged so that when key S1 is pressed the set input of the first bistable (pin 4) goes high. The associated Q-output at pin 2 also becomes high. When this has taken place, pressing key S2 results in the input of the second bistable (pin 6) and the associated Q-output (pin 9) going high. Thereafter, pressing keys S3 and S4 have similar results. This means that if keys S 1-S4 have been pressed in the correct sequence, a logic high level exists at the Q-output (pin 1) of the fourth bistable. If during this process one of the other keys (S5-S10) is pressed, the .set bistables are reset via R9 to make the code even more difficult to break. When pin 1 of IC1 is high. transistor Ti is switched on. whereupon the relay that operates the door lock is energized.
Before key S1 is operated. the Q-output of the first bistable is low so that its reset input is held low via R1, D1, and R5. The potential across C1 is then almost equal to the supply voltage. When S1 is pressed, pin 2 goes high. Because of D1, the ca will discharge slowly via R5 about seven seconds, the level at the reset et 8 inputs of all four bistables, for the present purpose, becomes logic high. If at that instant the relay is actuated, it will be de-energized. If the complete code has not been keyed in, a fresh start must be made. The reset time may be lengthened by increasing the value of capacitor C1.
A keyboard with more or fewer switches may be used. Fewer than six makes breaking code easy, however. If more than 10 switches are available, simply wire the additional ones in parallel with S5-S10. The code may be enlarged to eight digits by connecting a second 4043 in series with ICI. Pin 1 is then linked to key S5 and the following three keys are connected to the second 4043 in the same way as S2-S4 to IC1.
There is a slight drawback that has to be borne in mind: when switches S1-S4 are pressed simultaneously, the lock will open. This may be prevented by (a) making one of the keys S5-S10 a reset switch, and (b) not choosing adjacent keys for S1-S4.
In quiescent operation, the circuit draws a current_ of about 10 mA.